1. Field of the Invention
This invention relates to a system and a method for controlling the fuel supply to an internal combustion engine in which the amount of fuel supplied to an internal combustion engine is controlled by the output of an intake air sensor which operates to sense the amount of intake air sucked into the engine per intake stroke.
2. Description of the Related Art
Conventionally, fuel supply to an internal combustion engine is controlled based on the amount of intake air sucked into the engine per intake stroke which is calculated from the output of an intake air sensor (hereinafter abbreviated as AFS), which is disposed in an intake pipe at a location upstream of a throttle valve, as well as from the number of revolutions per minute of the engine.
In the case where an AFS is disposed in an intake pipe upstream of a throttle valve for sensing the amount of intake air sucked into an engine cylinder, the AFS measures, in addition to the amount of intake air actually sucked into the engine cylinder, the amount of intake air which is to be filled into a portion of the intake pipe between the throttle valve and the engine cylinder when the throttle valve is rapidly opened. Therefore, the AFS senses an amount of intake air greater than that actually sucked into the engine cylinder so that if fuel supply is controlled based on the output of the AFS, an air and fuel mixture supplied to the engine cylinder tends to become overrich.
In order to avoid such a situation, it was proposed to control the fuel supply by using the amount of intake air AN(n) sucked into the engine cylinder during the nth intake stroke (i.e., during the period between the nth and (n-1)th predetermined crank angle). In this case, AN.sub.(n) is determined by the following equation: EQU AN.sub.(n) =K.sub.1 .times.AN.sub.(n-1) +K.sub.2 .times.AN.sub.(t)
where AN.sub.(n-1) is the amount of intake air sucked into the engine cylinder during the (n-1)the intake stroke (i.e., during the period between the (n-1)th and (n-2)th predetermined crank angle); AN.sub.(t) is the output of the AFS (i.e., the amount of intake air which is sensed by the AFS at a predetermined crank angle of the engine); and K.sub.1 and K.sub.2 are coefficients of filteration for AN.sub.(n-1) and AN.sub.(t), respectively. Such control on fuel supply is to smoothe out the amount of intake air sucked into the engine cylinder on each intake stroke every time the engine takes a predetermined crank angle so as to effect proper control on fuel supply at all times especially at the time of rapid accelerations.
In the above-mentioned fuel control system, however, there is the following drawback. To modify the amount of intake air as sensed by the AFS necessarily creates a time lag in the calculation more than one intake stroke. Also, at the time of engine deceleration, there will be a time lag in the sensed output of the intake air sensor due to the presence of air in the intake pipe so that the amount of fuel supplied to the engine cylinder becomes excessive. Specifically, a portion of the fuel injected from a fuel injector adheres to the inner surface of the intake pipe and the remaining portion of the fuel is sucked into the engine cylinder. Accordingly, the amount of fuel forming an air/fuel mixture, which is to be sucked into the engine cylinder on a particular intake stroke, is the sum of a portion of fuel injected from the fuel injector on that intake stroke and a fuel which was previously supplied from the fuel injector on previous intake strokes and adhered to the inner surface of the intake pipe. In this connection, it is to be noted that the greater the engine load, the more is the amount of fuel supplied from the fuel injector so that the amount of fuel adhering to the intake pipe increases in proportion to the increasing engine load. In addition, the higher the number of revolutions per minute of the engine, the number of intake strokes per unit time increases so that the number of engine cycles having excessive fuel supply increases. Accordingly, the probability of excessive fuel supply becomes higher in accordance with an increase in the engine load and/or the number of revolutions per minute of the engine.